An airborne wind turbine is a design concept for a wind turbine that is supported in the air without a tower. Airborne wind turbines may operate in low or high altitudes. Since the generator is aloft, a conductive tether is used to transmit energy to the ground and to provide the required mechanical strength. Airborne turbine systems have the advantage of tapping an almost constant wind, without requirements for slip rings or yaw mechanisms, and without the expense of tower construction.
These flying electric generators are proposed to harness kinetic energy in the powerful, persistent high altitude winds. As shown in “Harnessing High Altitude Wind Power” by Bryan W. Roberts, et al., the average power density can be as high as 20 kW/m2 in an approximately 1000 km wide band around latitude 30° in both Earth hemispheres. Starting at 3000 feet altitude and above, tethered rotorcraft could give individual rated outputs from hundred of kilowatts up to several megawatts (MW). These aircraft are highly controllable and can be flown in arrays, making them a source of reliable wind power. Roberts shows an electromechanical tether designed to transmit 240kW at a voltage of 15 kV. The electrical transmission efficiency is 90%. The tether has two insulated aluminum conductors embedded in a Vectran fiber composite. The tether's specific weight is around 115 kg/km and has a diameter of 10 mm. The craft is designed for operations up to 15,000 feet (4600 m) and the total tether weight is over 500 kg. The weight of this tether is large compared with the air vehicle creating a suboptimum system.
As can be seen, there is a need for a tether for an airborne wind turbine that may conduct the generated electricity to the Earth, while providing shielding and lightning protection and strength.